Cookware heater

ABSTRACT

A cookware heater and method of manufacture thereof is presented wherein a heating circuit of the printed circuit foil type is bonded to a ceramic or metallic cookware item. A layer of high temperature dielectric material is sandwiched between the heater circuit foil and the cookware item, and the coefficients of expansion of the cookware material, and the heater circuit foil are closely matched to each other.

United States Patent [191 Howie 1 COOKWARE HEATER [75] Inventor: John R. Howie, Killingworth, Conn.

[73] Assignee: Safeway Products lnc., Middletown,

Conn.

[22] Filed: Sept. 28, 1973 [21] Appl. No.: 401,681

[52] US. Cl 219/438, 117/212, 219/464,

219/543 [51] Int. Cl. F27d 11/02 [58] Field of Search 106/52, 39; 117/212;

[56] References Cited UNITED STATES PATENTS 2,603,740 7/1952 Del Buttero 219/438 X 2,859,321 11/1958 Garaway 219/543 X 2,939.807 6/1960 Needham 117/212 Mar. 4, 1975 3,086,101 4/1963 SCofield 219/460 3,191,004 6/1965 Hocker 219/461 3,496,336 2/1970 Hingoramy et a1. 219/464 3,505,498 4/1970 Shevlin 219/461 3,679,473 7/1972 Blatchford et a1. 117/212 3,781,528 12/1973 Schrewelius 219/546 FOREIGN PATENTS OR APPLICATIONS 462,942 l/1950 Canada 219/438 Primary E.\'aminerVolodymyr Y. Mayewsky [57] ABSTRACT A cookware heater and method of manufacture thereof is presented wherein a heating circuit of the printed circuit foil type is bonded to a ceramic or metallic cookware item. A layer of high temperature dielectric material is sandwiched between the heater circuit foil and the cookware item, and the coefficients of expansion of the cookware material, and the heater circuit foil are closely matched to each other.

9 Claims, 5 Drawing Figures PY ROCE RAM ADHESIVE FIBERFRAX H EATER SUBSTRATE PATENIEDIIIR 4% I 3,869,596

STRIP GLAZE s my: F Ji i1; r i ADHERE DIELECTRIC /5 F/G4 I I Had BOND g L HEATER FOIL Z4 L CURE D POST CURE 5 7/ H PYROCERAM /8 ADHESIVE FIBERFRAX ADHESIVE h n m m m HEATER CIRCUIT FOIL. Z1 2 HEATER SUBSTRATE COOKWARE HEATER BACKGROUND OF THE INVENTION Cookware of the glazed pottery or ceramic type is a very popular item in widespread use. One of the most popular types of such cookware is the type known as Corningware (Corningware being a registered trademark of Coming Glass Works), a ceramic cookware item made from a type of ceramic material known as Pyroceram (Pyroceram being a registered trademark of Corning Glass Works). Similar utensils, containers and cookware items for kitchen use are made of porcelain, clay, stoneware, earthenware, and specially prepared glass or other similar heat resistant substances. Many of these ceraiii ic or glass items have a glazed finish, and they are commonly used to hold food during the course of cooking, such as in an oven or on a stove, and for service after cooking. However, while metal cookware having self-contained resistance wire heating elements have been known for some time, no satisfactory selfcontained. heating configuration for these ceramic cookware items has heretofore been constructed. Furthermore, the metal 1 cookware items with selfcontained heating elements have typically been of the heatingrod or coil type wherein the rod or coil is spaced-.from the cooking surface which results in a waste of aconsiderable amount of energy.

J" SUMMARY OF THE INVENTION In the present invention an electrically heated ceramic cookware unit is formed wherein a heating unit is bonded to the cooking plate, the heating unit containing a layer or coating of high temperature dielectric material bonded to the outer surface of the ceramic cookware unit and a printed circuit type foil heater circuit bonded to the layer of dielectric material. For those ceramic units such as the Corningware utensils which normally have a glazed outer surface, the glaze is removed in the area of the cooking plate at which the layers of dielectric and printed circuit are bonded. The coefficient of expansion of the circuit foil is closely matched to the coefficient of expansion of the material from which the cookware unit is formed. The layer of dielectric material may either be a coating applied directly to the cookware unit, or it may be a separate thin layer of relatively flexible material adhesively bonded to the ceramic cookware unit. The printed circuit heating element is adhesively bonded to the dielectric layer with the resistive foil adjacent to the dielectric material. The adhesive material is a thermosetting adhesive which retains some flexibility after curing. Since the adhesive and the dielectric are adhesive, matching of their coefficients of expansion to the foil and cookware unit is not critical.

The assembled unit is subjected to a curing cycle at a pressure range of from psi to 100 psi at a temperature ranging from 250F to 350F for a time ranging from 60 minutes to minutes. After the curing cycle has been completed, the unit is post cured for a minimum of two hours at a temperature at least equal to the normal end use anticipated operating temperature which is on the order of 450F.

The apparatus and method of the present invention can also be applied to cooking utensils of metallic construction whereby the standard rod or coil heating unit can be replaced with a printed circuit foil heating element. To bond the foil heating element to a metal cooking utensil, the bottom'of the metal cooking utensil is first spray coated with a layer of porcelain or other ceramic material to provide a mat finish. A layer of dielectric material and the printed circuit heating element'arethen bonded to the unit in the same manner and construction as described'above with respectto the ceramic cooking utensils.

Accordingly, one object of the present invention is to provide a novel and improved cooking unit and method of formation thereof.

Still another object of the present invention is to provide a novel and improved ceramic cooking unit and the method of formation thereof.

Still another object of the present invention is to provide a novel and improved ceramic cooking unit and method of formation thereof, particularly a cooking unit made of pyroceramic material.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings:

FIG. 1 is a perspective view of a cookware unit in accordance with the present invention.

FIG. 2 is a block flow diagram depicting the method of forming the cookware unit of the present invention.

FIG. 3 is a partial sectional view, taken along line 3-3 of FIG. 1, showing a detail of the construction of the cookware unit of the present invention.

FIG. 4 is a plan view illustrating a typical configuration or pattern of the heating element of the present invention.

FIG. 5 is an electrical schematic of the heating circuit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description of the preferred embodiments, the invention will be described in terms of a heating circuit bonded to Pyroceram ceramic material commercially distributed in the form of Corningware cooking utensils and dishes. However, it will be understood that the invention is generally applicable to various utensils and containers made of material generally designated as ceramic, porcelain, clay, stoneware, earthenware, or specially prepared glasses or similar heat-resistant substances, all of which will be referred to generally as ceramic hereinafter. A specific application to metallic pots or pans will also be discussed. The Pyroceram ceramic material is generally known and identified by various physical properties such as the following: dielectric constant (at room temperature and 1 MHz) 5.5 6.3; loss factor (at room temperature and 1 MHz) 0.01-0.07; dielectric strength (VPM) 250300; specific gravity 2.4.- 2.6; thermal conductivity (BTU/HR/sq. ft/F/ft.) l.l 2.1; thermal coefficient of expansion (l0" in./in./C) 0.2 4. In particular, the general purpose Pyroceram ceramic material No. 9608 used in making cooking utensils has a thermal coefficient of expansion of 0.2-1.1 X l0 in./in./F (0.36-1.98 X l0 in./in./C). Reference is hereby made to U.S. Pat. No. 2,920,971 to Stookey for purposes of incorporation herein of pertinent material and data relating to Pyroceram and Corningware, that patent having been identified in published reports as covering the Pyroceram material.

Referring now to FIG. 1, a cookware unit 10 is shown which has a generally dish-shaped rectangular body 12 of Pyroceram 9608 ceramic material. An electrical cord 14 extends from a-printed circuit heating element bonded to the bottom'or'base 16 of the cookware unit.

. Referringnow to'FlG. 3, a partial sectional view of "the base lb of the cookware unit isshown. The view of 3 ,i s an exploded view showing the. various adherenti 'layerswhich. form the heatingassembly bonded to theexterior surface .of the bottom of the cookwareunit.

The ty pical'ceramic cookwareunit has. a glazed outer 'surface,,and this'gla zledouter surfacein the area where the heater. is to be mountedis removed to present a mat finished surfaceJSat the exterior of base 16. A layer ofhigh'temperature dielectricmaterial 20 is bonded by adhesive-22 to mat surface .18, and a printed circuit heating element consisting of a heater circuit foil 24 mountedon a substrate 26 is bonded by adhesive 28 to dielectric layer 20, and hence to the cookware unit.

Cord 14 is connected toheater circuit foil 24- in any desired fashion topass an electric current through the circuit foil for the purpose of heating the cookware unit.

3 While the material from which the cookware unit is formed is generally considered to be an electrical insulating material, substantial drop-offs, in the dielectric strength at the standard operating temperature of around 450F has been noted, and the dielectric layer 20 is incorporated to electrically insulate the printed Y 'circuit heating element from the main body of the then the coating will be bonded directly to mat surface 18 and adhesive layer 22 can be eliminated.

The ceramics, such'as Pyroceram, generally used and available for cookware utensils have a low thermal coefficientof expansion. Itis of extreme importance in theifpresent invention that the thermal coeff cients of expansion of heater circuit foil material 24 be reasonablyclosely matched to the thermal coefficient of expansion of the base material from which the cookware unit formed. Thefoil material should have a higher th e'rmal-coefficientof expansion than the base materia'l', with an upper limit of a factor of 10. That is,the coefficient of thermal expansion of the foil material should be not less than nor more than 10 times the coefficient of thermal expansion of the base, material. Circuitfoil material having a thermal coefficient of expansionof 2.3-3.2 l'in;/in./C (1.56-1.78 x roam.- lihQ/F?) meets this relationship and is commercially available. Both the adhesive andthe dielectric layer (if inft h'e; form of a separate sheet) have a certain amount offlexibility and can thus accommodate expansion by physical distortion and so need not be ,as closely matched. Theadhesive is of the type which remains flexible after curing so it can accommodate a certain amount ,ofdifferential expansion between the. adhesive parts and between itself and the adhered parts.

' or by adhesively bonding a sheet of insulating material. In step C the printed, circuit heating element is mounted on the dielectric layer. of applying the adhesive layer 28 and then applying .thep'rinted circuit heating element to the adhesive layer 28. In both step C and instep B if adhesive layer 22 is employed, theadhesive is a high'temperatur'e thermosettingiadhesive material, and nominal coatings of 0.003 inches are used. h I

At this pointin thefformation process, the dielectric layer and the heating element are only lightly bonded to the bottom of the cookware: unit, and an appropriate curing process must be undertaken to complete the bonding and formation of the finished unit. T,o, that end, curing is accomplished instep D wherein the unit is curedat a pressure ranging from 15 to psi at a temperature ranging from approximately" 250F to 350F for a time period of from 60 minutes at the lower end of the temperature range to 20 minutes at the higher end of the temperature range. After the curing in step D, a post cure is employed in step E wherein the cured heater assembly is post cured fora minimum of 2 hours at a temperature at least equal to the normal end use anticipated operating temperature of the cooking unit which is about 450F. As can be seen in'FlG. 3, the heater substrate material 26, which is typically a fiberglass material to which the heater circuit foil is bonded, is on the exposed side of thefmal assembly. A protective coating may be placed over heater'substrate 26 if desired, or the assembled cooking unit may be mounted within a mating base, if desired, for both appearance and protective purposes.

Referring now to FIG. 4, a typical pattern is shown for heater circuit 24. Three general areas are indicated by the dotted outlines 30, 32-and 34, corresponding, respectively, to a control area, and an edge area of the heater unit. The circuitpattern is arranged to provide three different power densities, the highest power density being in control area 30, the second highest power density being in'edge area 34, and the lowest power density being in the central area 32. The highest power density ispro'vided in control area 30 tocompensate for heat losses to'a heat collector for a thermostatcontrol or other control device mounted in area 30 to control the temperature of the cookware unit. The second highest power density is established in edge area 34 .(only one half of whichis outlined in FIG. 4) tocompensate for edge, and side losses. Since heatlosses are lowest in the centralarea, the lowest power density is established in the center. The configuration shown and described with respect to FIG. 4 results in essentially uniform heat distribution across the entire bottom surface of the cookware unit. v

Referring now to FIG. 5,.a simple schematic of the electrical circuit is shown. Power density areas .30, 32 and 34 are also shown, and it can be seen that a pair of parallel circuits are used to form the edge areas, V

In accordance with the general concepts of the present invention, the heating element can also be mounted on a metallic cooking element such as a standard frying pan. In this embodiment, it has been found desirable to. first spray coat the bottom of the metal cookingutensil with a porcelain or ceramic or other similar material to obtain an. electrically insulating mat finish. Thus, the glazestripping step of step A is replaced with step A (shown in phantom in FIG. .3) wherein a porcelain layer is spray coated onto the exterior bottom of the metal cooking utensil. The remaining steps B, C, D and E are then completed as described with respect tothe ceramic cooking utensil. Accordingly, whether the cooking unit is of the preferred ceramic type or has a metallic body, the final configuration is as shown and described with respect to FIGS. 1 and 3, with the only difference being in the material of the body and in the spray coating of a layer of porcelain for the metal utensil rather than stripping the glaze for the ceramic materials.

All of the previous discussion has been directed to a pan type cookware unit. However, it is to be understood that the present invention applies equally well to a flat sheet of material which may form part of a stove or counter top, in which embodiment the foil heating elements would be bonded, as taught herein, to the underneath side of the flat sheet.

While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

What is claimed is:

l. A cooking unit including:

a cooking element formed of material selected from the group consisting of glass, ceramic and metal;

a base portion part of said cooking element having a mat surface and being electrically insulating;

a layer of dielectric material bonded to said mat surface of the base portion;

electro-conductive circuit foil means adhesively bonded to said layer of dielectric material, the coefficient of thermal expansion of said foil means being not less than the coefficient of the thermal expansion of said base portion and not more than ten times the coefficient of thermal expansion of said base portion; and

a layer of adhesive material bonding said circuit foil means to said layer of dielectric material, said adhesive remaining flexible after curing.

2. A cooking unit as in claim 1 wherein:

said layer of dielectric material is a discrete sheet of flexible material adhesively bonded to said base portion.

3. A cooking unit as in claim 2 including:

a layer of adhesive material bonding said dielectric sheet to said base, said layer of adhesive material between said base portion and said dielectric sheet being of the same material as the layer of adhesive material between said dielectric sheet and said circuit foil means.

4. A cooking unit as in claim 1 wherein:

said cooking element is formed of metal; and

said base portion is a layer of electrically insulating material applied to said metal. I

5. A cooking unit as in claim 4 wherein: said electrically insulating material is porcelain.

6. A cooking unit including a cooking element formed of material selected from the group consisting of glass and ceramic;

a base portion part of said cooking element having a mat surface;

a layer of dielectric material bonded to said mat surface of the base portion;

electro-conductive circuit foil means adhesively bonded to said layer of dielectric material, the coefficient of thermal expansion of said foil means being not less than the coefficient of thermal expansion of said cooking unit material and not more than ten times the coefficient of thermal expansion of said cooking unit material; and

a layer of adhesive material bonding said circuit foil means to said layer of dielectric material, said adhesive remaining flexible after curing.

7. A cooking unit as in claim 6 wherein:

said layer of dielectric material is a discrete sheet of flexible material adhesively bonded to said base portion.

8. A cooking unit as in claim 7 including:

a layer of adhesive material bonding said dielectric sheet to said base, said layer of adhesive material between said base portion and said dielectric sheet being of the same material as the layer of adhesive material between said dielectric sheet and said circuit foil means.

9. A cooking unit as in claim 5 wherein:

said layer of dielectric material is a sheet of aluminasilica fiber material.

I 'UNETED PATENT OFFICE CERTIFICATE OF CORRECTION YATENT NOr 3,869,596 DATED ':,Ma,rch-.4, 1975- INVENTOHS) John R. Howie 1t is certified that error appears in the above-identified patent and that said Letters Patent are hereby c'orrectedas shown below:

Column 4, line 33, "after "a:

area" was omitted.

' Claim 9 should be dependent upon claim 6 rather than on claim 5.

Signed and sealed this 20th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH c. MASON v Commissioner of Patents Attesting Officer and Trademarks ea," ,the phrase "the central Patent No. 5, 9,59 Dated March A, 1975 John R. Howie Page 1 of 2 Inventor(s It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Fig. 3, change "PYROCERAM" to GLASS-CERAMIC Column 1, rewrite lines 6-10 as, popular brands of such cookware is that known as Corning Ware, a cookware made from a glass-ceramic material (Corning Ware being a registered trademark of Column 1, line 37, change "Corningware" to Corning Ware Column 2, line 17, change "pyroceramic" to glass-ceramic Column 2, line 38, change "Pyroceram ceramic" to a glass-ceramic Column 2, line 39, change "in the form of Corningware" to as Corning Ware Column 2, line &8, change "Pyroceram ceramic" to glass-ceramic Column 2, line 56, change "Pyroceram ceramic" to glass-ceramic Column 2, line 62, change "Pyroceram and Corningware" to the glassceramic material Inventor(s) John R. Howie Page 2 of 2 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,869,596 Dated March It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 6h, change "Pyroceram" to glass-ceramic Column 2 line 6 amic 7, change Pyroceram 9608 ceramlc" to the 9608 glass- Column 3, line 36, change "Pyroce'ram" to the glass-ceramics Signed and Scaled this Third Day Of January 1978 [SEAL] Arrest:

RUTH C. MASON I LUTRELLE F. PARKER A ttesting Officer Acting Commissioner of Patents and Trademarks 

1. A COOKING UNIT INCLUDING: A COOKING ELEMENT FORMED OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF GLASS, CERAMIC AND METAL; A BASE PORTION PART OF SAID COOKING ELEMENT HAVING A MAT SURFACE AND BEING ELECTRICALLY INSULATING; A LAYER OF DIELECTRIC MATERIAL BONDED TO SAID MAT SURFACE OF THE BASE PORTION; ELECTRO-CONDUCTIVE CIRCUIT FOIL MEANS ADHESIVELY BONDED TO SAID LAYER OF DIELECTRIC MATERIAL, THE COEFFICIENT OF THERMAL EXPANSION OF SAID FOIL MEANS BEING NOT LESS THAN THE COEFFI-
 2. A cooking unit as in claim 1 wherein: said layer of dielectric material is a discrete sheet of flexible material adhesively bonded to said base portion.
 3. A cooking unit as in claim 2 including: a layer of adhesive material bonding said dielectric sheet to said base, said layer of adhesive material between said base portion and said dielectric sheet being of the same material as the layer of adhesive material between said dielectric sheet and said circuit foil means.
 4. A cooking unit as in claim 1 wherein: said cooking element is formed of metal; and said base portion is a layer of electrically insulating material applied to said metal.
 5. A cooking unit as in claim 4 wherein: said electrically insulating material is porcelain.
 6. A cooking unit including a cooking element formed of material selected from the group consisting of glass and ceramic; a base portion part of said cooking element having a mat surface; a layer of dielectric material bonded to said mat surface of the base portion; electro-conductive circuit foil means adhesively bonded to said layer of dielectric material, the coefficient of thermal expansion of said foil means being not less than the coefficient of thermal expansion of said cooking unit material and not more than ten times the coefficient of thermal expansion of said cooking unit material; and a layer of adhesive material bonding said circuit foil means to said layer of dielectric material, said adhesive remaining flexible after curing.
 7. A cooking unit as in claim 6 wherein: said layer of dielectric material is a discrete sheet of flexible material adhesively bonded to said base portion.
 8. A cooking unit as in claim 7 including: a layer of adhesive material bonding said dielectric sheet to said base, said layer of adhesive material between said base portion and said dielectric sheet being of the same material as the layer of adhesive material between said dielectric sheet and said circuit foil means.
 9. A cooking unit as in claim 5 wherein: said layer of dielectric material is a sheet of alumina-silica fiber material. 